Militants attacked one of Iraq's largest air bases and seized control of several small oilfields today as US military experts arrived to set up an operations centre to help Iraqi security forces counter a mounting Sunni insurgency.

Militants overran the Ajeel oil site, 30 km (19 miles) east of Tikrit, which contains at least three small oilfields that produce 28,000 barrels per day, an engineer working at the field said.

The engineer said local tribes had taken responsibility for protecting the fields after police withdrew but that they also left after the nearby town of al-Alam was seized by militants.

State TV showed troop reinforcements flying into the compound by helicopter to fend off the assault on Baiji, a strategic industrial complex 200 km north of Baghdad.

Insurgents have also surrounded a massive air base nearby, which was known as "Camp Anaconda" under US occupation, and struck it with mortars. Eyewitnesses said the air base had been surrounded on three sides.

Iraqi Prime Minister Nuri al-Maliki, who is fighting for his job and is under international pressure to create a more inclusive government, said he supported starting the process of forming a new government within a week.

In northern Iraq the Sunni militants extended a two-week advance that has been led by the hardline Islamic State in Iraq and the Levant (ISIL) but also includes an amalgam of other Sunni groups angered by Maliki's rule.

They blame him for marginalising their sect during eight years in power. The fighting threatens to rupture the country two and a half years after the end of US occupation.

US Secretary of State John Kerry pressed Iraqi officials to form an "inclusive" government during a visit this week and urged leaders of the autonomous Kurdish region to stand with Baghdad against the onslaught.

A parliament session is planned within a week that will start the process of forming a new government based on the results of elections held in April.

US President Barack Obama has offered up to 300 advisers to Iraq, about 130 of whom have now been deployed.

Rear Admiral John Kirby, the Pentagon press secretary, said late yesterday an initial group sent to establish an operations centre included intelligence analysts, logistics experts and special operations forces.

Another 50 US military personnel working in the region are expected to arrive within the next few days to create four additional assessment teams, he said. US military personnel are also flying regular manned and unmanned reconnaissance flights over Iraq.

Iraqi state television reported that newly-arrived Pentagon advisers met with Baghdad's operations commander and agreed to set up a joint operation command.

The country is racing against time as the insurgents consolidate their grip on Sunni provinces.

The United Nations says more than 1,000 people, mainly civilians, have been killed during the Sunni insurgents' advance in Iraq, spearheaded by al Qaeda offshoot ISIL.